1. Field of the Invention
The present invention relates to an improvement on an image forming apparatus utilizing a light-emitting diode (LED).
2. Description of the Related Art
Various image forming apparatuses utilizing LEDs have already been in practical use, but an LED head employed in such apparatuses has a limited depth of focus in a condensing lens, and, with a defocus of ±50 μm, may show a deterioration in the focusing ability, thus resulting in a dispersion of light. In such case, a condensed light spot formed on a surface of a photosensitive drum by the light condensed with the lens becomes larger in size, thereby possibly causing a change in the density of a halftone image.
FIGS. 10A and 10B show toner images formed on the surface of the photosensitive drum, by irradiating the surface of the photosensitive drum with the condensed light spot and then supplying a toner according to a predetermined procedure. FIGS. 10A and 10B show a halftone image formed by condensed light spots in a checkerboard pattern, taking 2×2 pixels as a spot, in which a broken-lined square represents a pixel.
FIG. 10A shows a state where the condensed light spot is in focus, in which the toner images have a space therebetween as anticipated, thereby providing a halftone image of an appropriate density. On the other hand, FIG. 10B shows a state where the condensed light spot is defocused and becomes somewhat larger. Thus, the toner images become connected between mutually near portions thereof, whereby the space shown in FIG. 10A becomes smaller. As a result, the halftone image has a density higher than a desired density.
In order to solve the drawback associated with the defocus of the condensed light spot, the JP-A-2004-25678 discloses a technology of employing a motor for displacing the LED head in a focusing direction and displacing the LED head according to a set value entered by an operator thereby regulating the focus point to an optimum position.
Also JP-A-9-174932 discloses a technology of placing a light controlling film between a light-emitting part and a lens in the LED head to suppress a light spreading, thereby relaxing the defocus.
However such prior technologies involve drawbacks of complicating the structure of the LED head, thereby elevating the production cost.